The present invention relates to an electrical field absorbing semiconductor optical modulator, more particularly, to a high speed semiconductor optical modulator and a fabricating method thereof.
A semiconductor laser is mainly used as a light source of optical communication. Once current is applied to a semiconductor laser, a monochromatic light is irradiated. Optical signals are transmitted by modulating intensity of the monochromatic light and by having the modulated light pass through an optical fiber. Conventionally, a method of modulating the current applied to a semiconductor laser is used to modulate the intensity of a light emitted from a semiconductor laser
However, such method generates xe2x80x9cChirpingxe2x80x9d, which is xe2x80x9cdisplacement of a light wavexe2x80x9d, thereby causing troubles in sending a long-distant high speed signal over 2,5 Gb/s.
As a solution of the above trouble, proposed is a technique of outputting a semiconductor laser light having a predetermined intensity and modulating intensity of a light by using another optical device outside. The optical device playing such a role is called an optical modulator which includes an electric-field absorbing type optical modulator, etc.
The optical modulator of an electric-field absorbing type use the phenomenon that a band gab is shrunk provided that an electric field is applied to a semiconductor laser. Namely, a light passes through the optical modulator when an electric field is applied to the semiconductor laser. On the other hand, the optical device blocks the light by absorbing once the electric field is applied to. Thus, the optical device produces an optical signal by modulating a certain intensity of the light transmitted from a light source.
The optical modulator of the electric-field absorbing type is manufactured to have a figure of a wave-guiding passage by using a semiconductor of a direct transition having a band gap amounting to a light wave emitted from a semiconductor laser or by using a multi-quantum well(MQW).
FIG. 1 shows a cross-sectional structure of an optical modulator of an electric-field absorbing type according to a related art.
Referring to FIG. 1, after a first conductive type InGaAsP layer 12, an MQW absorbing layer 13, a second conductive type InGaAsP layer 14, a second conductive type InP layer 15, and a second conductive type ohmic contact layer 16 have been formed on a substrate 11 successively, all layers except a belt pattern are etched to expose a surface of the substrate 11.
Then, polyimide 17 is formed around the belt pattern on the substrate 11. And, metal electrodes 19 and 20 are formed on a top of the belt pattern and beneath the substrate 20.
As a refractive index of the MQW absorbing layer 13 is higher than that of the surroundings, an incident light into the MQW layer 13 progresses along with the wave-guiding passage of the belt pattern, thereby being absorbed continuously or transmitted to the reverse direction of incidence.
Once a bias voltage is applied to a p-type electrode 19 and an n-type electrode 20 which are formed on the top of and under the optical modulator respectively, that is, voltage of the n-type electrode 20 is higher than that of the p-type electrode 19, carriers are generated from the MQW absorbing layer 13. Thus, electrons proceed to the n-type electrode 20, while holes move to the p-type electrode 20.
On the other hand, the light transmits through the MQW absorbing layer 13 as there occurs no absorption of light without applied voltage thereto.
Such optical modulator of an electric-field absorbing type modulates the intensity of light by changing the voltage applied to the p-type and n-type electrodes 19 and 20.
Important parameters of the optical modulator are modulation speed and extinction ratio. And, variables deciding the modulation speed are capacitance, resistance, charge transportation, and the like which are related to the structure of the optical modulator.
In order to reduce capacitance, the MQW absorbing layer is formed thin and/or the area of the MQW layer should be small. Thus, the area of the absorbing layer should be reduced because high speed optical modulation of tens of giga needs capacitance under 0.1 pF. Therefore, width of the absorbing layer is limited under 2 xcexcm.
Capacitance may be decreased provided that the MQW absorbing layer is formed thick. But, this technique cancels out the effect of decreasing capacitance since high speed operation is inhibited by increased operating voltage and slow transportation speed.
Unfortunately, contact resistance between the electrode and the MQW absorbing layer increases due to the narrow width of the MQW absorbing layer in the optical modulator according to the related art. The increased contact resistance slows down the speed of optical modulation. That is why a new structure reducing both capacitance and contact resistance is required for the fabrication of a high speed optical modulator.
Accordingly, the present invention is directed to a high speed semiconductor optical modulator and a fabricating method thereof that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
The object of the present invention is to provide a high speed semiconductor optical modulator and a fabricating method thereof which form a narrow width of a light absorbing layer and a wide electrode contact area simultaneously in order to reduce both capacitance and contact resistance hindering the high speed operation of a device.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention includes a high speed semiconductor optical modulator, the optical modulator formed by stacking an n-type light-wave guiding layer, a light absorbing layer, a p-type light-wave guiding layer, a p-type clad layer, and a p-type ohmic contact layer on a substrate successively, the optical modulator having a ridge structure wherein the optical modulator is an electric-field absorbing type, and wherein width W3 of the light absorbing layer is less than the width W1 of the p-type ohmic contact layer.
Preferably, widths of the n-type light-wave guiding layer, p-type light-wave guiding layer, and p-type clad layer are identical one another and wherein the width W2 is longer than or equal to the width W1 of the p-type ohmic contact layer. And, the width W3 of the light absorbing layer is 1-3 xcexcm while the width of the p-type ohmic contact layer is 5-10 xcexcm.
In another aspect, the present invention includes the following steps. A first step is of forming a ridge structure by stacking an n-type light-wave guiding layer, a light absorbing layer, a p-type light-wave guiding layer, a p-type clad layer, and a p-type ohmic contact layer on a substrate successively. A second step is of etching the p-type ohmic contact layer to have a width W1. A third step is of etching vertically the n-type light-wave guiding layer, the light absorbing layer, the p-type light-wave guiding layer, the p-type clad layer, and the p-type ohmic contact layer to have widths W2. And a fourth step is of side-etching the light absorbing layer to have a width W3 which is narrower than the width W1.
The present invention further includes the steps of a fifth step of forming a dielectric at a side of the ridge structure, a sixth step of forming a dielectric film on the p-type clad layer while the p-type ohmic contact layer is exposed, and a seventh step of forming a p-type electrode on the p-type ohmic contact layer and an n-type electrode beneath the substrate.
Preferably, the fourth step is carried out during a predetermined time enough to etch completely a test pattern of which width is equal to a difference between the widths W2 and W3.
And, the width W2 is equal to or wider than the width W1 of the p-type ohmic contact layer while the width W3 of the light absorbing layer is 1xcx9c3 xcexcm and the width W1 of the p-type ohmic contact layer is 5xcx9c10 xcexcm.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.